Wick structure and system and method of making wick structure

ABSTRACT

In a method of forming a porous wick structure, a space is filled with particles formed from electrically conductive material such that the particles contact one another at contact points within the space. The particles are compressed within the space at a pressure of less than 0.1 MPa. While compressing the particles, electrical current is imparted through the particles in to simultaneously form bonds between adjacent particles at points of contact between the particles. The bonds fix the particles in position with respect to one another such that pores are defined between adjacent particles. A porous wick structure can be formed by this method.

GRANT STATEMENT

This work is financially supported by NASA Cooperative Agreement Notice, Grant Number 80NSSC18M0030.

FIELD

The present disclosure generally relates to wick structures suitable for use in heat transfer applications, and in particular, to a method of forming a wick structure by passing electrical current through a plurality of conductive particles that are in contact with one another without substantially compacting the particles.

BACKGROUND

It is known to form wick structures by bonding together particles such that the particles are fixed in place with respect to one another to define a fluidly interconnected network of pores between the particles. Such wick structures have numerous applications in science and industry. For example, wick structures can passively pump liquid toward a heated surface by capillary flow. Wick structures are used in both open-loop and closed-loop (e.g., heat pipe) heat transfer applications, to provide cooling to electronic devices, power generation equipment, lasers, nuclear plants, and the like.

Although porous bodies in general and wick structures in particular have numerous applications, they can be difficult to manufacture. In the typical process, the particles are placed into a mold and then heated in a furnace for an extended period of time until heat bonds are formed between the particles. This process is known as “furnace sintering.” Furnace sintering requires heating substantially the entire volume of the particles and thus takes a long time and consumes a large amount of energy.

SUMMARY

In one aspect, a method of forming a wick structure comprises filling a space with particles formed from electrically conductive material such that the particles contact one another at contact points within the space, compressing the particles within the space at a pressure of less than 0.1 MPa, and while compressing the particles, imparting electrical current through the particles to simultaneously form bonds between adjacent particles at points of contact between the particles. The bonds fix the particles in position with respect to one another such that pores are defined between adjacent particles.

In another aspect, a wick structure is formed by the above-described method.

Other aspects will be apparent hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a system for forming a wick structure;

FIG. 2 is a photograph of two copper wick structures formed from 400-micron particles using a prototype of the system;

FIG. 3 is a magnified photograph of one of the wick structures of FIG. 2;

FIG. 4 is another magnified photograph of one of the wick structures of FIG. 2;

FIG. 5 is a photograph of a stainless steel wick structure formed from 1/32″ stainless steel ball bearings using a prototype of the system of FIG. 1.

Corresponding reference characters indicate corresponding parts throughout the drawings.

DETAILED DESCRIPTION

Referring to FIG. 1, a system for forming a wick structure is generally indicated at reference number 10. The system broadly includes a mold, generally indicated at 12, and an electrical current source (broadly, an electrical energy source), generally indicated at 14. As will be explained in further detail below, the electrical current source 14 is configured to pass a large pulse of electrical energy through electrically conductive particles P that are compressed in a cavity 16 (broadly, a space or defined space) of the mold 12. Due to electrical resistance at the points of contact between the particles P, the electrical current passing through the particles nearly instantaneously generates a large amount of localized heat at the points of contact. As explained in further detail below, the system 10 uses the heat to bond the particles P together at the points of contact without substantially compacting the particles together. The bonded particles form a highly porous unitary body that can function as a wick structure.

The above-described system and process is related to the system and process for electro sinter forging described in U.S. Pat. No. 9,227,244, which is hereby incorporated by reference in its entirety. U.S. Pat. No. 9,227,244 contemplates applying pulses of high current electrical energy to very fine powders that are simultaneously compacted under very high pressure to forge solid bodies of high-density or medium-density material. The present inventors have recognized that, by controlling the current and pressure applied to the particles in a manner contrary to what is taught in U.S. Pat. No. 9,227,244, high porosity bodies can be formed with characteristics that are suitable for use as wick structures.

Referring again to FIG. 1, the illustrated mold 12 comprises a side wall 20 that extends longitudinally along an axis and circumferentially about the perimeter of the mold cavity 16. In an embodiment, the side wall 20 is formed from an electrically insulating material, e.g., a ceramic material or a dielectric. The illustrated mold 12 further comprises an upper end plate 22 (broadly, a first end plate) and a lower end plate 24 (broadly, a second end plate) spaced apart along the axis of the side wall 20. In the illustrated embodiment, the lower end plate 24 forms a bottom of the mold 12. More specifically, the lower end plate 24 has an upper recess that forms a bottom of the mold cavity 16. The side wall 20 sits on a short rim that extends circumferentially around the upper recess. Each end plate 22, 24 is formed from an electrically conductive material.

At least one of the end plates 22 is operatively connected to a press mechanism, generally indicated at 26. In the illustrated embodiment, the press mechanism 26 is operatively connected to the upper plate 22 for moving the upper plate toward the lower plate 24 to compress the particles P within the mold cavity 16 while the system 10 applies electrical current to the particles. The upper plate 22 thus functions as a push plate or ram for applying pressure to the particles P during the wick-forming process. In certain embodiments, the press mechanism can simultaneously move both of the end plates toward one another to compress the particles during the wick-forming operation. As will be explained in further detail below, the pressure that the press mechanism 26 imparts on the particles is substantially less than the pressure imparted by the electro sinter forging system disclosed in U.S. Pat. No. 9,227,244. The inventors have recognized that the particles can still be bonded together when minimal compression forces are applied, and further, that bonding the particles together in this fashion can facilitate forming highly porous bodies (e.g., bodies having porosities of greater than about 0.4) that are suitable for use as wick structures. Any suitable mechanical or electromechanical device may be used for the press mechanism 26 to move at least one end plate 22 toward the other 24 at a controlled force. For example, in one or more embodiments, the press mechanism 26 comprises one of a pneumatic cylinder, a hydraulic cylinder, an electronic solenoid, or other type of linear actuator.

In the illustrated embodiment, the electrical current source 14 is schematically illustrated to be a simple capacitor. However, it will be understood that any electrical circuitry that is suitable for delivering one or more pulses of electrical energy to the particles can be used without departing from the scope of this disclosure. For example, it is expressly contemplated that the same general electrical circuitry may be used that is disclosed in U.S. Pat. No. 9,227,244. For example, the electrical current source can comprise a capacitor bank and a step-down transformer that is configured to reduce the voltage and increase the current of the electrical energy discharged from the capacitor bank before applying it to the particles P.

In the illustrated embodiment, a first electrode 32 provides electrical communication between the electrical current source 14 and the upper plate 22 and a second electrode 34 provides electrical communication between the electrical current source and the lower plate 24. Providing the plates 22, 24, between the electrodes 32, 34 and the particles P is thought to enable the application of even pressure on the particles, even heat release, and minimal adhesion to the surfaces applying pressure to the particles. In one or more embodiments, each of the plates is formed from a material that is electrically conductive and defines as chemically inert surface to contact the particles P in the mold. This enables the plates 22, 24 to pass current to the particles P without bonding to them. Although the illustrated embodiment includes electrodes 32, 34 that are separately attached to the plates 22, 24 (which bear directly on the particles P), in one or more embodiments the plates can be omitted such that that electrodes bear directly on the particles (e.g., one or both electrodes can act as ram as disclosed in U.S. Pat. No. 9,227,244).

In an embodiment, each of the electrodes 32, 34 has a relatively large cross-sectional area in relation to the cross-sectional area of the mold cavity 16. The optimal cross-sectional area of the electrodes will depend on factors such as energy/power, weld time, electrode force, welding current, panel contact resistance, surface contact area, mold dimensions, minimum melt energy, and alternating current frequency when alternating current is applied. The illustrated mold cavity 16 has a maximum cross-sectional dimension (e.g., inner diameter) D1 and the electrode has a tip cross-sectional dimension (e.g., an outer diameter) D2 that is at least 75% of the maximum cross-sectional dimension of the mold cavity. Although FIG. 1 depicts the electrode diameter D2 as being slightly less than the mold cavity diameter D1, it will be understood that the electrode diameter could be equal to or greater than the mold cavity diameter in one or more embodiments. In an exemplary embodiment, the electrode tip diameter D2 is slightly greater than the mold cavity diameter Di, e.g., in an inclusive range of from about 101% to about 200% of the mold cavity diameter, from about 101% to about 150% of the mold cavity diameter, from about 101% to about 125% of the mold cavity diameter, from about 101% to about 110% of the mold cavity diameter.

An exemplary method of using the system 10 to form a wick structure will now be described. In one or more embodiments, the system 10 is located in an inert fluid environment during the process of forming the wick structure. The inert fluid environment can minimize oxidation of the particles P.

Initially, the technician imparts the desired volume, mass, or quantity of particles P into the mold cavity 16. In an embodiment, the lower plate 24 remains in position at the lower end of the mold cavity 16 so that the particles P pile onto the lower plate. The particles P flow freely into the mold cavity 16 and contact one another at contact points within the mold 12. Suitable particles for forming wick include, but are not limited to, copper particles, copper alloy particles, silver particles, gold particles, aluminum nitride particles, nickel particles, stainless steel particles, tungsten particles, zinc particles, or aluminum particles. In certain embodiments, the particles have a particle size in an inclusive range of from 50 μm to about 500 μm.

After placing the particles P into the mold cavity 16, the first plate 22 is positioned to contact the upper layer of particles in the mold cavity. The first plate 22 closes the upper end of the mold 12 so that no particles can escape the mold during the remainder of the wick-forming process.

To bond the particles together, the press mechanism 26 urges the upper plate 22 toward the lower plate 24, thereby compressing the particles P between the plates. The press mechanism 26 controls the force at which the upper plate 22 is urged toward the lower plate 24 to apply a pressure of less than 0.1 MPa to the particles P. With the particles P are slightly compressed in this manner, the electrical current source 14 discharges one or more electrical pulses that pass through the electrodes 32, 34 and the plates 22, 24, to the particles P in the mold 12. In one or more embodiments, the electrical current source is configured to discharge electrical current in an inclusive range of from about 100 A to about 200 A to the particles P. The voltage of the electrical energy can be in an inclusive range of from about 5 V to about 25 V. In an embodiment, the electrical current source 14 is configured to discharge a single continuous direct current pulse that is the sole electrical energy imparted to the particles P to bond the particles together into a wick structure. Multiple direct current pulses or alternative current pulses are also possible in one or more embodiments.

The process can be defined in terms of three discrete time intervals: a squeeze time, an electrical discharge time which follows the squeeze time, and a hold time which follows the electrical discharge time. During the squeeze time, pressure is applied to the particles without applying current. During the electrical discharge time, pressure and current are simultaneously applied to the particles. And during the hold time, pressure is applied to the particles without applying current. Upon completion of the hold time, the particles, now bonded together to form a wick structure, are released from the mold 12.

As the electrical current passes through the particles P in the mold 12 during the electrical discharge time, the current generates localized heat at the contact points due to the large electrical resistance at the interfaces between the particles. The localized heat quickly bonds the particles together at the contact points. Because the particles are held together by relatively low pressure and the heat is substantially localized at the contact points, the particles are not substantially compacted while the current is being applied. For example, in one or more embodiments, neither the shapes nor sizes of the particles P are substantially altered during the wick forming process. In addition, the particles P generally retain the same relative positions that occur when the particles flow freely into the mold cavity 16. As a result, the inherent pores that are present between particles when the particles are allowed to flow freely into a container are substantially retained after the particles are bonded together. In other words, the pores before bonding and after bonding are almost the same size and shape, with only localized surface spot bonding occurring. This surface spot bonding ensures that the maximum porosity is achieved with the high particle adhesive integrity and strength, which further enhances the durability and longevity of the wick structure, while amplifying the thermal transport phenomena. The bonded particles thus form a highly porous structure comprising a tortuous network of fluid channels extending through the body. In one or more embodiments, the bonded particles P form a body having a porosity of at least about 0.4 (e.g., at least about 0.45, at least about 0.5, at least about 0.55, at least about 0.6). Accordingly, the bonded particles form a porous body that is suitable for use as a wick structure.

The above-described system and process can form wick structures in substantially less time and using substantially less energy than conventional furnace-based systems and processes for forming wick structures. Whereas the primary mechanism in traditional furnace sintering is localized melting along the particle interface, phase change, localized mass transport across the interface of the touching particles thereby reducing the localized particle homogeneity based on elemental composition of the parent material, using the illustrated particle bonding system, the primary mechanism is the interfacial interlocking and intertwining due to spot softening with the assistance of thermal effects and electrode pressures, resulting in a lesser interfacial melting and re-solidification in comparison with traditional furnace sintering. Thus, it is believed that the above-described system and process can be used to make wick structures more widely accessible and less costly. Further, in comparison with wick structures formed in conventional furnace-based systems, less of the microstructure of the material forming a wick structure made using the system 10 is affected by the heat of the particle-bonding operation.

EXAMPLES

A basic prototype of the above-described system 10 has been constructed and tested to demonstrate the efficacy of the system for forming wick structures. In the prototype system, a Chicago Electric Spot Welder (CESW) was utilized as the electrical current source 14. The CESW is powered by 120 volts AC, has 6-inch tongs with pointed tips standard, draws 13.5 amps, and generates output voltage of 1.5 kVA at 50% duty cycle. The CESW is normally used for welding sheet metal. In normal use, the CESW can spot weld two pieces of 20 gauge (galvanized) steel of combined thickness of ⅛ inches.

The standard 6-inch tongs of the CESW have pointed tips. In the prototype system, the standard pointed tips were replaced with polished leveled tips (e.g., flat tips) in order to form electrodes with greater surface area to prevent localized melting. In particular, the pointed end of each standard tip was removed and a molybdenum disc of 0.25-inch thickness was joined to each end to form the electrodes 32, 34 of the prototype system. The diameter of the molybdenum disc was 0.625″, which is the same as the standard diameter of the CESW tong before tapering to the pointed tip.

To enable the CESW to pressurize the particles during the wick-forming operation as described above, the tongs were further fitted with 3D-printed weight supports. These weight supports were loaded with weight until a desired pressure between the tongs was achieved (e.g., a pressure of less than 0.1 MPa).

A wick mold was formed from a ceramic tube having an outer diameter of 0.625″ and an inner diameter of 0.5″. The inner perimeter of the ceramic tube defined the mold cavity 16 of the prototype system.

To test the prototype system, a thin copper disc was placed on one molybdenum electrode, the ceramic tube was placed on the disc, and the interior of the ceramic tube was filled with 400-micron copper particles. The upper tong was weighted down to pressurize the particles between the molybdenum electrodes at a pressure of less than 0.1 MPa. Current was then discharged from the CESW to join the particles together. Referring to FIGS. 2-4, the resulting wick from the above example method is shown. The wick was made using 400-micron copper particles to create a 0.5″ wick. The porosity of the example was 41%.

Referring to FIG. 5, a similar prototype system was used to form another wick structure from 1/32″ stainless steel balls. The porosity of this wick was 38%.

When introducing elements of the present disclosure or the preferred embodiment(s) thereof, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of the elements. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements.

In view of the above, it will be seen that the several objects of the disclosure are achieved and other advantageous results attained.

As various changes could be made in the above products and methods without departing from the scope of the disclosure, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense. 

What is claimed is:
 1. A method of forming a wick structure, the method comprising: filling a space with particles formed from electrically conductive material such that the particles contact one another at contact points within the space; compressing the particles within the space at a pressure of less than 0.1 MPa while compressing the particles, imparting electrical current through the particles in to simultaneously form bonds between adjacent particles at points of contact between the particles, wherein the bonds fix the particles in position with respect to one another such that pores are defined between adjacent particles.
 2. The method as set forth in any of claim 1, wherein the step of passing an electrical current through the particles is performed in an inert fluid environment.
 3. A wick structure formed by performing the method of claim
 2. 4. The wick structure as set forth in claim 3, wherein the wick structure has a porosity of greater than 0.4.
 5. A wick structure formed by performing the method of claim
 1. 6. The wick structure as set forth in claim 5, wherein the wick structure has a porosity of greater than 0.4. 